Most metal alloys undergo a volume contraction when they solidify. This volume contraction can result in a shrinkage porosity in a cast part unless an appendage, often called a "riser," of appropriate size is properly placed on the casting. The riser serves to feed the shrinkage of the metal in the mold, and is cut off after casting. Typical risers need to have volumes approaching the volume of the casting in order to produce a pore-free casting, leading to large recycling costs. This problem is particularly severe in high-purity castings, such as typical magnesium and magnesium alloy castings, because the riser may become contaminated during post-processing of the casting, and must be repurified before the metal can be used in another casting.
It is an object of the present invention to provide a method and apparatus for casting which allows riser volume to be substantially reduced.